Commerciallly available fluorescent lamps comprise an elongated tubular envelope having a pair of electrodes sealed into the opposite ends thereof. The envelope contains a gaseous atmosphere, which has a mixture of a rare gas and a metal vapor, such as mercury vapor. The interior surface of the envelope is coated with a finely-divided phosphor which is exposed to the electrical discharge between the two electrodes, and is excited by the ultraviolet radiations emitted by this discharge. The phosphor layer is usually applied by suspending a particulate phosphor material in a suitable binder, flushing the interior surface of the envelope with the suspension, permitting the excess suspension to drain out of the envelope, and then heating the interior surface of the coated envelope to a temperature which promotes adherence of the coating to the envelope interior surface and removes, generally by volatilization, the binder material. There results a phosphor layer adhered to the interior surface of the tubular envelope.
While the lamp is operating, the phosphor is in a mercury vapor discharge where it is subjected to ultraviolet radiation and bombardment by electrons and mercury atoms and ions. These factors may be responsible for lamp maintenance losses; i.e., for the time-dependent decrease in light output found in all fluorescent lamps compared to the original light output.
Various uses of alumina within fluorescent lamps have been proposed in an attempt to alleviate the deleterious effects of the short wavelength ultraviolet radiation and mercury vapor exposure. For example, U.S. Pat. Nos. 4,079,288 and 4,058,639, as well as others, discuss employing a layer of alumina on the interior surface of the fluorescent lamp envelope and applying phosohor thereon.
U.S. Pat. No. 3,886,396 teaches the application of a thin, porous, discontinuous layer of alumina being applied over the phosphor layer, and U.S. Pat. No. 2,386,277 teaches the application of a thin, nontransparent layer of alumina being applied over the phosphor layer also. While all of these techniques provide some benefits of varying degrees, none of them improve the initial lamp brightness.